Conventionally, there has been used a quadrature modulator for the quadrature modulation of an I signal and a Q signal, namely baseband signals. A quadrature modulator is provided with an I signal multiplier which receives the I signal, a Q signal multiplier which receives the Q signal, and an adder. The I signal multiplier multiplies the I signal and a local signal for the I signal, and outputs a result of the multiplication. The Q signal multiplier multiplies the Q signal and a local signal for the Q signal (signal obtained by shifting the local signal for the I signal by 90 degrees), and outputs a result of the multiplication. The adder adds the output of the I signal multiplier and the output of the Q signal multiplier, and outputs a result of the addition. The output of the adder is an RF (Radio Frequency) signal obtained as a result of the quadrature modulation of the baseband signals.
However, the quadrature modulator presents initial offset errors. Namely, the RF signal presents errors in the phase and the amplitude.
In order to eliminate the errors in the phase and the amplitude generated in the RF signal, a patent document 1 (Japanese Laid-Open Patent Publication (Kokai) No. 2000-69097) describes a configuration where one quadrature modulator is additionally provided to eliminate the errors. Namely, an output of the quadrature modulator used for the error elimination is added to an output of the quadrature modulator used for the quadrature modulation to eliminate the errors in the phase and the amplitude generated in the RF signal.
However, upon using the above error elimination method, it is necessary to use two quadrature modulators. The multiplier provided for the quadrature modulator is expensive, and a cost to eliminate the errors thus increases accordingly.
In view of the foregoing problem, an object of the present invention is to correct offset errors of a quadrature modulator and the like without additionally providing a quadrature modulator for error elimination.